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How to Use an Air Balance Hood

While an air-balancing hood is the most popular instrument used to verify the performance of an HVAC system, it could never get the job done alone. Those who use a hood once or twice by itself can easily lose confidence in it. The art of using these test instruments together to verify system performance requires a significant sustained commitment to testing. Good science requires more than a single test in almost every situation.

When measuring the performance of an HVAC system, an air-balancing hood is the go to test instrument for top technicians. However, in a short period of time each of us will learn that a balancing hood is only a single tool in an arsenal of HVAC diagnostic testing weapons and that reading grilles and registers are only one piece of the diagnostic puzzle.

Air balancing used to be a term reserved for the testing and adjusting of commercial HVAC systems. Today, in the residential and light commercial HVAC industry, the term air balancing is more accurately applied to HVAC System Diagnostics, Field Measured-Performance Verification, or Installed Efficiency Documentation.

While an air-balancing hood is the most popular instrument used to verify the performance of an HVAC system, it could never get the job done alone. Those who use a hood once or twice by itself can easily lose confidence in it. This happens because they fail to pay the price to learn how it works together with its test instrument cousins: the manometer, anemometer, thermometer and psychrometer.

Art and Science

The art of using these test instruments together to verify system performance requires a significant sustained commitment to testing. Good science requires more than a single test in almost every situation.

The first encounters with an air-balancing hood can leave a new tester bewildered. Most of us have built HVAC systems for years and are certain they work perfectly. I’ve seen grown men taking their first airflow test on a return grille expecting 1000 CFM become discouraged when it only read 600 CFM. They called the hood a piece of junk and walked away disgusted.

Developing Trust in Your Instruments

Initially, we all trusted our systems, and distrusted our test instruments. But eventually, you’ll learn to couple the hood reading with a total external static pressure test, and then take some pressure drop readings. Someday you’ll discover a .53-in. w.c. pressure drop through a “high-efficiency” filter. Then you’ll measure return duct temperatures over 100 degrees and calculate Btu deliveries at less than 50% of rated equipment capacity. Necessity eventually drives you to advanced testing requiring you to take airflow traverse readings used to calculate correction factors for certain hood tests.

Through sustained testing, the day comes when you read airflow and your first thought becomes “What’s wrong with my system?” rather than “What’s wrong with the hood?” We call this air balancing puberty. Welcome to the club.

Testing Challenges

One of the first challenges you’ll wrestle with is the constantly changing air volume through grilles. Steady state testing doesn’t exist in the field. We are in the business of heat transfer – moving heat from one place to another. The very nature of moving heat around assures testing will be affected by a constantly changing set of conditions. A register may vary from 190 CFM to 210 CFM within a few seconds. This makes airflow readings appear challenging, but it’s a reality of how air moves through a system. Understanding constantly changing test conditions improves your confidence in your readings and magnifies your ability to interpret them.

So, when the numbers jump around a little on your hood, don’t be discouraged. Either get an average of the readings, or consistently use the high reading and you’ll be just fine. You’re now beginning to understand how air really works.

The primary fault in air balance hood testing is failing to get a tight seal between the hood and the wall, floor, or ceiling that the grille is attached to. Some registers require larger skirts to completely capture all the airflow. Various sizes are available and have to be used from time to time. You’ll also use foam rubber adaptors that allow you to capture the airflow from baseboard grilles and other unusual installations.

Many registers pose testing challenges by their very nature. Toe-kick grilles cannot be read with a hood unless gaskets are used to direct the airflow from the register through the hood. There are a number of methods that can be used to adapt a hood to gain consistent, verifiable readings under adverse test conditions.

All Hoods Are Not Created Equal

There are some “disposable” hoods on the market that deliver poor, unreliable test numbers that probably shouldn’t be offered to the industry. Because of the low cost (often around a thousand dollars) they are frequently picked up to quickly balance a contractor’s first commercial job. They are widely used in low-income weatherization programs also. Bottom line with these hoods; you get what you pay for. Spend your money on a real balancing hood that you can trust.

On the other end of the scale are quality made and calibrated commercial hoods. When used correctly, maintained well, and kept in calibration, they deliver solid readings you can count on. Good commercial hoods cost around $2,500 or more. Obtaining good readings with any hood requires a working knowledge of measuring system performance. This often demands additional testing to verify airflow readings taken with an air-balancing hood related to system readings with other instruments.

One frequently quoted report from outside the industry compared a number of air balancing hoods in a pitiful attempt to discredit the use of these instruments. The “scientific findings” included hoods that have never been available for purchase to the public, and applied average readings from all the hoods to evaluate the entire collection of instruments. Air-balancing hoods are extremely dependable and are the go-to test instrument for those who have paid the price to learn how to use them.

The market offers a number of balancing hoods, discuss your purchase with a balancing tech that can recommend a hood that they know is reliable and avoid the temptation to save a few bucks.

But Wait, There’s More

I had a recently certified air balancing tech call in. It was hot and he was tired and trying to get performance maximized on a cooling system at the end of the day. He had only tested airflow and static pressure and thought he was done. I asked for his temperature readings, his enthalpy change through the system, his fan RPM, refrigerant pressures and temperatures, and some engineering data from the manufacturer’s equipment specifications. Then I needed to know total Btu delivery and his installed Cooling System Efficiency Ratio (CSER™).

Finally he sighed deeply and said, “Doc, there has to be some shortcuts to verify the performance of an HVAC system!” I replied, “Sorry, we’ve got to look at the whole system. There are no effective shortcuts.” So he worked late and called me in the morning and announced he had achieved 97% cooling capacity from the system. He apologized for being a sissy the night before and was pleased with his work once he got some rest.

Verifying HVAC system performance with balancing hoods and related air balance testing to prove total installed system performance is certainly the future of our industry. With the upcoming changes in minimum equipment efficiency, the next frontier is testing the duct system and quantifying installed system performance.

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company and membership organization. If you're an HVAC contractor or technician interested in a free Balancing Hood Test Procedure, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.